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In 1993, there was a well control incident while drilling a prospect offshore Vietnam. This incidentnecessitated the implementation of an extensive seismic monitoring programme. It became rapidly apparent that conventional high resolution seismic techniques for identifying and monitoring gas in the subsurface were inadequate due to loss of signal power through the system. This problem drove the need for an innovative approach to acquiring a 'Medium Resolution 3D Suwey'. This had two important characteristics, namely significantly higher bandwidth than conventional 3D data and also having the penetration necessary in order to image the target reservoir, through a disturbed overburden. This paper describes the technology used in acquiring and processing the data, as this has relevanceto use of this type of acquisition for conventional reservoir description purposes. It also describes the attribute analysis developed to compare multiple vintages of seismic in order to be able to track fluid movement in the subsurface. The enhanced resolution achieved has relevance to enabling better reservoir characterisation in general. The attribute analysis is an example of how these type of data can then be conditioned in order to allow tracking of fluid in the subsurface, in appropriate situations. Introduction/background Using seismic data to track fluid movement is a fairly well known technique, but one that still needs considerable development for routine use. In certain situations the rock properties of an interval change significantly enough to create a measurably different seismic response. [n order to apply this technology it is necessary to establish an understanding ofThe distribution of rock properties in the interval of interestThe scale of the change in seismic response caused by different fluid fills (water, oil, gas)Whether the quality of the available seismic data has the amplitude, time and depth resolutions required to detect these changesAnd, for monitoring cases, whether the quality of the seismic is controlled sufficiently to enable comparisons between successive surveys In the case presented here, the interest was in monitoring possible subsurface gas movements following a well control incident. The technical problems and the approach, however, are quite generic; in that the same principles apply for fluid monitoring experiments in conventional production situations. For the particular problem presented here, it was necessary to establish the history of gas movement in the subsurface using a series of seismic data sets which had different acquisition and processing histories. The need was to understand to what extent it was possible to measure the extent of gas charged sands using these seismic data sets.

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